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Efros AL, Brus LE. Nanocrystal Quantum Dots: From Discovery to Modern Development. ACS NANO 2021; 15:6192-6210. [PMID: 33830732 DOI: 10.1021/acsnano.1c01399] [Citation(s) in RCA: 122] [Impact Index Per Article: 40.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
This review traces nanocrystal quantum dot (QD) research from the early discoveries to the present day and into the future. We describe the extensive body of theoretical and experimental knowledge that comprises the modern science of QDs. Indeed, the spatial confinement of electrons, holes, and excitons in nanocrystals, coupled with the ability of modern chemical synthesis to make complex designed structures, is today enabling multiple applications of QD size-tunable electronic and optical properties.
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Affiliation(s)
- Alexander L Efros
- Center for Computational Material Science, Naval Research Laboratory, Washington, DC 20375, United States
| | - Louis E Brus
- Department of Chemistry, Columbia University, New York, New York 10027, United States
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2
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Zhou Q, Zhou H, Tao W, Zheng Y, Chen Y, Zhu H. Highly Efficient Multiple Exciton Generation and Harvesting in Few-Layer Black Phosphorus and Heterostructure. NANO LETTERS 2020; 20:8212-8219. [PMID: 33044075 DOI: 10.1021/acs.nanolett.0c03328] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Multiple exciton generation (MEG) in semiconductors that yields two or more excitons by absorbing one high-energy photon has been proposed to break the Shockley-Queisser limit and boost photon-to-electron conversion efficiency. However, MEG performance in conventional bulk semiconductors or later colloidal nanocrystals is far from satisfactory. Here, we report efficient MEG in few-layer black phosphorus (BP), a direct narrow bandgap two-dimensional (2D) semiconductor with layer-tunable properties. MEG performance improves with decreasing layer number and reaches 2.09Eg threshold and 93% efficiency for two-layer BP, approaching energy conservation limit. The enhanced MEG can be attributed to strong Coulomb interaction and high density of states in 2D materials. Furthermore, MEG of BP shows negligible degradation in vertical heterostructure and multielectron can be extracted by interfacial transfer with near unity yield. These results suggest 2D semiconductors as an ideal system for next generation highly efficient light emission and charge transfer devices.
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Affiliation(s)
- Qiaohui Zhou
- State Key Laboratory of Modern Optical Instrumentation, Centre for Chemistry of High-Performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang China
| | - Hongzhi Zhou
- State Key Laboratory of Modern Optical Instrumentation, Centre for Chemistry of High-Performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang China
| | - Weijian Tao
- State Key Laboratory of Modern Optical Instrumentation, Centre for Chemistry of High-Performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang China
| | - Yizhen Zheng
- State Key Laboratory of Modern Optical Instrumentation, Centre for Chemistry of High-Performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang China
| | - Yuzhong Chen
- State Key Laboratory of Modern Optical Instrumentation, Centre for Chemistry of High-Performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang China
| | - Haiming Zhu
- State Key Laboratory of Modern Optical Instrumentation, Centre for Chemistry of High-Performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang China
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Li M, Begum R, Fu J, Xu Q, Koh TM, Veldhuis SA, Grätzel M, Mathews N, Mhaisalkar S, Sum TC. Low threshold and efficient multiple exciton generation in halide perovskite nanocrystals. Nat Commun 2018; 9:4197. [PMID: 30305633 PMCID: PMC6180109 DOI: 10.1038/s41467-018-06596-1] [Citation(s) in RCA: 51] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2018] [Accepted: 09/09/2018] [Indexed: 11/09/2022] Open
Abstract
Multiple exciton generation (MEG) or carrier multiplication, a process that spawns two or more electron–hole pairs from an absorbed high-energy photon (larger than two times bandgap energy Eg), is a promising way to augment the photocurrent and overcome the Shockley–Queisser limit. Conventional semiconductor nanocrystals, the forerunners, face severe challenges from fast hot-carrier cooling. Perovskite nanocrystals possess an intrinsic phonon bottleneck that prolongs slow hot-carrier cooling, transcending these limitations. Herein, we demonstrate enhanced MEG with 2.25Eg threshold and 75% slope efficiency in intermediate-confined colloidal formamidinium lead iodide nanocrystals, surpassing those in strongly confined lead sulfide or lead selenide incumbents. Efficient MEG occurs via inverse Auger process within 90 fs, afforded by the slow cooling of energetic hot carriers. These nanocrystals circumvent the conundrum over enhanced Coulombic coupling and reduced density of states in strongly confined nanocrystals. These insights may lead to the realization of next generation of solar cells and efficient optoelectronic devices. The hot carriers in halide perovskite nanocrystals cool much slower than those in conventional semiconductor nanocrystals due to the phonon bottleneck. Here, Li et al. demonstrate enhanced multiple exciton generation with lower threshold in intermediate-confined perovskite nanocrystals based on this effect.
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Affiliation(s)
- Mingjie Li
- School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore, Singapore
| | - Raihana Begum
- Energy Research Institute @ NTU (ERI@N), 50 Nanyang Drive, Research Techno Plaza, X-Frontier Block, Level 5, 637553, Singapore, Singapore
| | - Jianhui Fu
- School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore, Singapore
| | - Qiang Xu
- School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore, Singapore
| | - Teck Ming Koh
- Energy Research Institute @ NTU (ERI@N), 50 Nanyang Drive, Research Techno Plaza, X-Frontier Block, Level 5, 637553, Singapore, Singapore
| | - Sjoerd A Veldhuis
- Energy Research Institute @ NTU (ERI@N), 50 Nanyang Drive, Research Techno Plaza, X-Frontier Block, Level 5, 637553, Singapore, Singapore
| | - Michael Grätzel
- Laboratory of Photonics and Interfaces, Department of Chemistry and Chemical Engineering, Swiss Federal Institute of Technology, Station 6, CH-1015, Lausanne, Switzerland
| | - Nripan Mathews
- Energy Research Institute @ NTU (ERI@N), 50 Nanyang Drive, Research Techno Plaza, X-Frontier Block, Level 5, 637553, Singapore, Singapore.,School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore, Singapore
| | - Subodh Mhaisalkar
- Energy Research Institute @ NTU (ERI@N), 50 Nanyang Drive, Research Techno Plaza, X-Frontier Block, Level 5, 637553, Singapore, Singapore. .,School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore, Singapore.
| | - Tze Chien Sum
- School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore, Singapore.
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Stolle CJ, Lu X, Yu Y, Schaller RD, Korgel BA. Efficient Carrier Multiplication in Colloidal Silicon Nanorods. NANO LETTERS 2017; 17:5580-5586. [PMID: 28762274 DOI: 10.1021/acs.nanolett.7b02386] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Auger recombination lifetimes, absorption cross sections, and the quantum yields of carrier multiplication (CM), or multiexciton generation (MEG), were determined for solvent-dispersed silicon (Si) nanorods using transient absorption spectroscopy (TAS). Nanorods with an average diameter of 7.5 nm and aspect ratios of 6.1, 19.3, and 33.2 were examined. Colloidal Si nanocrystals of similar diameters were also studied for comparison. The nanocrystals and nanorods were passivated with organic ligands by hydrosilylation to prevent surface oxidation and limit the effects of surface trapping of photoexcited carriers. All samples used in the study exhibited relatively efficient photoluminescence. The Auger lifetimes increased with nanorod length, and the nanorods exhibited higher CM quantum yield and efficiency than the nanocrystals with a similar band gap energy Eg. Beyond a critical length, the CM quantum yield decreases. Nanorods with the aspect ratio of 19.3 had the highest CM quantum yield of 1.6 ± 0.2 at 2.9Eg, which corresponded to a multiexciton yield that was twice as high as observed for the spherical nanocrystals.
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Affiliation(s)
- Carl Jackson Stolle
- McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin , Austin, Texas 78712, United States
| | - Xiaotang Lu
- McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin , Austin, Texas 78712, United States
| | - Yixuan Yu
- McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin , Austin, Texas 78712, United States
| | - Richard D Schaller
- Department of Chemistry, Northwestern University , Evanston, Illinois 60439, United States
- Center for Nanoscale Materials, Argonne National Laboratories , Argonne, Illinois 60439, United States
| | - Brian A Korgel
- McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin , Austin, Texas 78712, United States
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Eshet H, Baer R, Neuhauser D, Rabani E. Theory of highly efficient multiexciton generation in type-II nanorods. Nat Commun 2016; 7:13178. [PMID: 27725668 PMCID: PMC5062596 DOI: 10.1038/ncomms13178] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2016] [Accepted: 09/09/2016] [Indexed: 12/20/2022] Open
Abstract
Multiexciton generation, by which more than a single electron–hole pair is generated on optical excitation, is a promising paradigm for pushing the efficiency of solar cells beyond the Shockley–Queisser limit of 31%. Utilizing this paradigm, however, requires the onset energy of multiexciton generation to be close to twice the band gap energy and the efficiency to increase rapidly above this onset. This challenge remains unattainable even using confined nanocrystals, nanorods or nanowires. Here, we show how both goals can be achieved in a nanorod heterostructure with type-II band offsets. Using pseudopotential atomistic calculation on a model type-II semiconductor heterostructure we predict the optimal conditions for controlling multiexciton generation efficiencies at twice the band gap energy. For a finite band offset, this requires a sharp interface along with a reduction of the exciton cooling and may enable a route for breaking the Shockley–Queisser limit. Multiple exciton generation could help limit thermalization losses in solar cells, but the efficiency of the process is still limited. Here, the authors show by atomistic calculations that type-II interfaces in nanostructures along with a change in exciton cooling rate favour multiple exciton generation.
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Affiliation(s)
- Hagai Eshet
- School of Chemistry, The Sackler Faculty of Exact Sciences, Tel Aviv University,Tel Aviv 69978, Israel.,The Raymond and Beverly Sackler Center for Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv 69978, Israel
| | - Roi Baer
- Fritz Haber Center for Molecular Dynamics, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
| | - Daniel Neuhauser
- Department of Chemistry, University of California at Los Angeles, Los Angeles, California 90095 USA
| | - Eran Rabani
- The Raymond and Beverly Sackler Center for Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv 69978, Israel.,Department of Chemistry, University of California and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
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ten Cate S, Sandeep CSS, Liu Y, Law M, Kinge S, Houtepen AJ, Schins JM, Siebbeles LDA. Generating free charges by carrier multiplication in quantum dots for highly efficient photovoltaics. Acc Chem Res 2015; 48:174-81. [PMID: 25607377 DOI: 10.1021/ar500248g] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
CONSPECTUS: In a conventional photovoltaic device (solar cell or photodiode) photons are absorbed in a bulk semiconductor layer, leading to excitation of an electron from a valence band to a conduction band. Directly after photoexcitation, the hole in the valence band and the electron in the conduction band have excess energy given by the difference between the photon energy and the semiconductor band gap. In a bulk semiconductor, the initially hot charges rapidly lose their excess energy as heat. This heat loss is the main reason that the theoretical efficiency of a conventional solar cell is limited to the Shockley-Queisser limit of ∼33%. The efficiency of a photovoltaic device can be increased if the excess energy is utilized to excite additional electrons across the band gap. A sufficiently hot charge can produce an electron-hole pair by Coulomb scattering on a valence electron. This process of carrier multiplication (CM) leads to formation of two or more electron-hole pairs for the absorption of one photon. In bulk semiconductors such as silicon, the energetic threshold for CM is too high to be of practical use. However, CM in nanometer sized semiconductor quantum dots (QDs) offers prospects for exploitation in photovoltaics. CM leads to formation of two or more electron-hole pairs that are initially in close proximity. For photovoltaic applications, these charges must escape from recombination. This Account outlines our recent progress in the generation of free mobile charges that result from CM in QDs. Studies of charge carrier photogeneration and mobility were carried out using (ultrafast) time-resolved laser techniques with optical or ac conductivity detection. We found that charges can be extracted from photoexcited PbS QDs by bringing them into contact with organic electron and hole accepting materials. However, charge localization on the QD produces a strong Coulomb attraction to its counter charge in the organic material. This limits the production of free charges that can contribute to the photocurrent in a device. We show that free mobile charges can be efficiently produced via CM in solids of strongly coupled PbSe QDs. Strong electronic coupling between the QDs resulted in a charge carrier mobility of the order of 1 cm(2) V(-1) s(-1). This mobility is sufficiently high so that virtually all electron-hole pairs escape from recombination. The impact of temperature on the CM efficiency in PbSe QD solids was also studied. We inferred that temperature has no observable effect on the rate of cooling of hot charges nor on the CM rate. We conclude that exploitation of CM requires that charges have sufficiently high mobility to escape from recombination. The contribution of CM to the efficiency of photovoltaic devices can be further enhanced by an increase of the CM efficiency above the energetic threshold of twice the band gap. For large-scale applications in photovoltaic devices, it is important to develop abundant and nontoxic materials that exhibit efficient CM.
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Affiliation(s)
- Sybren ten Cate
- Optoelectronic Materials Section, Department of Chemical
Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
| | - C. S. Suchand Sandeep
- Optoelectronic Materials Section, Department of Chemical
Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
| | - Yao Liu
- Department of Chemistry, University of California, Irvine, Irvine, California 92697, United States
| | - Matt Law
- Department of Chemistry, University of California, Irvine, Irvine, California 92697, United States
| | - Sachin Kinge
- Toyota Motor Europe, Functional Nanomaterials Lab, Advanced Technology, Hoge Wei 33, B-1930 Zaventem, Belgium
| | - Arjan J. Houtepen
- Optoelectronic Materials Section, Department of Chemical
Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
| | - Juleon M. Schins
- Optoelectronic Materials Section, Department of Chemical
Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
| | - Laurens D. A. Siebbeles
- Optoelectronic Materials Section, Department of Chemical
Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
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Sills A, Califano M. Origins of improved carrier multiplication efficiency in elongated semiconductor nanostructures. Phys Chem Chem Phys 2015; 17:2573-81. [DOI: 10.1039/c4cp03706e] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
Our calculations show that the origins of improved carrier multiplication efficiency in elongated semiconductor nanostructures can be attributed purely to electronic structure effects.
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Affiliation(s)
- Andrew Sills
- Institute of Microwaves and Photonics
- School of Electronic and Electrical Engineering
- University of Leeds
- UK
| | - Marco Califano
- Institute of Microwaves and Photonics
- School of Electronic and Electrical Engineering
- University of Leeds
- UK
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Abstract
In electronic structure theory, electron-electron repulsion is normally considered only in an average (or mean field) sense, for example, in a single Hartree-Fock determinant. This is the simple molecular orbital model, which is often a good approximation for molecules. In infinite systems, this averaging treatment leads to delocalized electronic bands, an excellent description of bulk 3D sp(3) semiconductors. However, in reality electrons try to instantaneously avoid each other; their relative motion is correlated. Strong electron-electron repulsion and correlation create new collective states and cause new femtosecond kinetic processes. This is especially true in 1D and 2D systems. The quantum size effect, a single electron property, is widely known: the band gap increases with decreasing size. This Account focuses on the experimental consequences of strong correlation. We first describe π-π* excited states in carbon nanotubes (CNTs). To obtain the spectra of individual CNTs, we developed a white-light, right-angle resonant Rayleigh scattering method. Discrete exciton transitions dominate the optical absorption spectra of both semiconducting and metallic tubes. Excitons are bound neutral excited states in which the electron and hole tightly orbit each other due to their mutual Coulomb attraction. We then describe more generally the independent roles of size and dimensionality in nanoelectronic structure, using additional examples from graphene, trans-polyacetylene chains, transition metal dichalcogenides, organic/inorganic Pb iodide perovskites, quantum dots, and pentacene van der Waals crystals. In 1D and 2D chemical systems, the electronic band structure diagram can be a poor predictor of properties if explicit correlation is not considered. One- and two-dimensional systems show quantum confinement and especially strong correlation as compared with their 3D parent systems. The Coulomb interaction is enhanced because the electrons are on the surface. One- and two-dimensional systems can exhibit essentially molecular properties even though they are infinite in size. Zero-dimensional Qdots show quantum confinement and modest electron correlation. Correlation is weak in 3D bulk semiconductors. Strongly correlated electronic states can behave as if they have fractional charge and effectively separate the spin and charge of the electron. This is apparent in the "soliton" state of polyacetylene, the fractional charge quantum Hall state of graphene, and the Luttinger electrical conductivity of metallic CNTs.
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Affiliation(s)
- Louis Brus
- Department of Chemistry, Columbia University, New York, New York 10027, United States
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